net: stmmac: Fix null-function call in ISR on stmmac1000

(resent due to overhelpful mail client corrupting patch)

At least on Meson GXBB, the CORE_IRQ_MTL_RX_OVERFLOW interrupt is thrown
with the stmmac1000 driver, which does not support set_rx_tail_ptr. With
this patch and the clock fixes, 1G ethernet works on ODROID-C2.

Signed-off-by: Matt Corallo <git@bluematt.me>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index a473c18..e407126 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -2804,7 +2804,7 @@
 				priv->tx_path_in_lpi_mode = true;
 			if (status & CORE_IRQ_TX_PATH_EXIT_LPI_MODE)
 				priv->tx_path_in_lpi_mode = false;
-			if (status & CORE_IRQ_MTL_RX_OVERFLOW)
+			if (status & CORE_IRQ_MTL_RX_OVERFLOW && priv->hw->dma->set_rx_tail_ptr)
 				priv->hw->dma->set_rx_tail_ptr(priv->ioaddr,
 							priv->rx_tail_addr,
 							STMMAC_CHAN0);